Solar Thermal Energy

Energy in the form of heat (thermal energy) is required for many purposes in the domestic, agricultural, industrial, and commercial sectors of the economy. Solar thermal energy can meet many of these needs in a cost-effective and reliable manner. A solar thermal system basically absorbs the heat in solar energy and transfers it to a medium such as water or air. When used for purposes such as heating of water, cooking, drying of various commodities and products, and distillation of water, solar thermal systems can partially or fully replace the conventional fuels such as coal, oil, and electricity.

Solar Water Heating Systems

A solar water heating system is a device that uses solar energy to heat water for domestic, commercial, and industrial needs. Heating of water is the most common application of solar energy in the world. A typical solar water heating system can save up to 1500 units of electricity every year, for every 100 litres per day of solar water heating capacity.

Design

A solar water heating system consists of a flatplate solar collector, a storage tank kept at a height behind the collector, and connecting pipes. The system is generally installed on the roof or on open ground, with the collector facing the sun and connected to a continuous water supply. The collector usually comprises copper tubes welded to copper sheets (both coated with a highly absorbing black coating) with a toughened glass sheet on top and insulating material on the back. The entire assembly is placed in a flat box. In certain models, evacuated glass tubes are used instead of copper; a separate cover sheet and insulating box are not required in this case. Water flows through the tubes, absorbs solar heat, and is stored in a tank. The hot water so stored can be used for various applications at homes, such as bathing, cleaning, and washing. It can also be used for a variety of industrial applications. The water stored in the tank remains hot overnight as the storage tank is insulated and heat losses are small.

Advantages of solar water heaters

• Hot water is available 24 hours a day depending on the use and system capacity.
• A solar water heater pays back its cost in 3–4 years.
• Solar water heaters last a long time (15–20 years) and require only simple maintenance.

Solar Cooking

Cooking is a common application of solar energy in India. A variety of solar cookers are available to suit different requirements. The MNES has been implementing a programme for promoting the use of solar cooking in the country for over two decades. The programme also supports the development and demonstration of new types of cooking systems.

Advantages of solar cooking

Solar Drying Systems

Many agricultural and industrial products need drying in order to reduce their moisture content as part of processing or for preservation. While open sun drying may be the most inexpensive and extensively used option for many of the products, the process is unhygienic and time-consuming. One option is to use a conventional fuel such as biomass, oil, or electricity for drying applications. However, such fuels are expensive, and their use causes pollution. With the rising costs of conventional fuels and increasing awareness of the dangers of pollution, solar dryers are becoming a technically and economically viable option in many industrial and agricultural applications.

Uses

Solar dryers can be utilized for various domestic purposes. They also find numerous applications in industries such as textiles, wood, fruit and food processing, paper, pharmaceutical, and agro-industries.

Technology

Solar energy can be used to heat air to temperatures needed for most of the drying applications. Solar dryers use air heated through solar energy collectors, which can be installed in modules according to the requirements of hot air. Drying is basically a heat and mass transfer process: moisture from the surface and inside of the product is vaporized, and the vapour is removed by flowing hot air. Important factors affecting the drying process are listed below.

1. Relative humidity and temperature of air
2. Airflow rate
3. Initial moisture content of the product
4. Final desired moisture content of the product

Types of solar dryers

Integrated solar dryers
An integrated solar dryer is one in which solar energy collection and drying take place in a single unit. Cabinet dryers, rack dryers, tunnel dryers, greenhouse dryers, and multi-rack dryers fall under this category. Normally, these dryers are small in size and are stand-alone units.

Distributed solar dryers
A solar dryer in which solar energy collection and drying take place in separate units is known as a distributed solar dryer. This type of solar dryer has two parts: (1) a flat-plate air heater and (2) a drying chamber. Air is heated in the flat-plate heater placed on the roof of the building or on the ground. Hot air from the air heater is circulated in the drying chamber with the help of a blower. These dryers can be designed in different sizes with various configurations, depending upon the temperature of hot air, airflow rate, types of products to be dried, etc.

Mixed-mode solar dryers
A solar dryer in which solar energy collection takes place in both air heater and drying unit, and drying takes place only in the drying unit, is known as a mixed-mode solar dryer. In this dryer, solar energy is collected through flat-plate solar collectors and also by the roof of the drying chamber. In large industrial drying systems, the solar-heated air is combined with air heated by conventional energy; this adds to the reliability of the system, and at the same time helps in significantly reducing conventional energy consumption.

Advantages

• Solar dryers are more economical compared to dryers that run on conventional fuels/electricity.
• The drying process is completed in the most hygienic and ecofriendly way.
  
• Solar drying systems have low operation and maintenance costs.
• Solar dryers last longer. A typical dryer can last 15–20 years with minimum maintenance.

Limitations

• Drying can be performed only during sunny days, unless the system is integrated with a conventional energy-based system.
• Due to limitations in solar energy collection, the solar drying process is slow in comparison with dryers that use conventional fuels.
• Normally, solar dryers can be utilized only for drying at 40–50 ºC.

Energy-Efficient Buildings

Technology
Buildings, as they are often designed and built today, contribute to serious environmental problems.
This is because energy intensive methods are used to construct a building and meet its demands for heating, cooling, ventilation, and lighting. These methods lead to severe depletion of the precious environmental resources. However, buildings can be designed in such a way that they meet the occupants’ needs for thermal and visual comfort at reduced levels of energy and resources consumption. Adopting an integrated approach to building design can bring about energy efficiency in new constructions.

The primary steps in this approach are listed below.

• Incorporate solar passive techniques in building design to minimize load on conventional systems (heating, cooling, ventilation, and lighting).
• Design energy-efficient lighting and HVAC (heating, ventilation, and air-conditioning) systems.
• Use renewable energy systems (solar photovoltaic systems/solar water heating systems) to meet a part of the building load.
• Use low energy materials and methods of construction, and reduce transportation energy.

In brief, an energy-efficient building balances all aspects of energy use in a building – lighting, space-conditioning, and ventilation – by providing an optimized mix of passive solar design strategies, energy efficient equipment, and renewable sources of energy.